The installation of a branch connection on a main pipe constitutes a point of weakness, particularly when the branch connection forms a heat exchanger with the fluid conveyed by the main pipe, there being large temperature differences between the fluid conveyed by the main pipe and the fluid of the branch connection.
In space applications for rocket engines where the engine is fed with propellant components at high pressure by means of turbopumps, such a heat exchanger, then referred to as a “hot” heat exchanger, may consist in heating liquid helium entering at a temperature of −263° C. to an outlet temperature close to 20° C. for operating requirements of the cryogenic engine.
Under such conditions, it will be understood that the branch connection, and in particular the stub mounted on the main pipe in order to make the branch connection, must be capable of withstanding extreme temperatures, and temperature differences that give rise to expansions or even structural modifications to the materials constituting the parts of the branch connection, which can lead to weakening of the mechanical connection between the parts.
As a general, rule, and as shown in FIG. 1, in order to make a branch connection on a main pipe 10, an annular flange 12 is used that is mounted by welding on the circular opening 14 of a collar 16 made in the wall of the main pipe 10. The opening 14 through which the branch connection is made to the main pipe 10 differs from the openings formed at each of the ends of the main pipe.
For this purpose, the collar 16 is made by panel beating techniques, i.e. by deforming the sheet metal constituting the wall of the main pipe 10. More precisely, rotary tools are used that successively perform an operation of piercing and cutting out an elliptical orifice, an operation of transversely stretching the material surrounding said orifice so as to constitute the collar 16, and then a turning operation with rotary cutters to obtain the final shape for the collar 16 and its circular opening 14.
Nevertheless, and in particular because of the stretching applied to the material to form the collar 16, the sheet metal becomes thinner in the collar. The collar 16 is also subjected to a concentration of stresses due to the pressure of the fluids and to external forces, thereby leading to thermal deformations that appear in the form of plastification and thus as a loss of elastic qualities that would enable the branch connection to remain intact during successive cycles.
It will be understood that modifying the qualities of this zone leads to weakness and to risks of breakage that are unacceptable.